To living organisms, oxygen is essential in supporting their life for example, in producing energy and in metabolism. Reactions in energy-producing systems, enzymatic reactions, and reactions induced by ultraviolet rays or radiations convert oxygen to he so-called active oxygen species, such as oxygen anion radical, peroxide ion and hydroxy radical. While the active oxygen species are of service to living organisms, promoting the function of oxygenase and the bactericidal activity of leukocytes, for instance, they promote the peroxidation of unsaturated fatty acids occurring abundantly in living organisms and constituting phospholipids in biomembranes, such as oleic acid, linolic acid, linolenic acid and arachidonic acid, causing formation of lipid peroxides. Like the above-mentioned active oxygen species, these lipid peroxides induce formation of alkoxy and hydroxy radicals, thus invading biomembranes and causing membrane disorders and deactivation of various enzymes [Taisha (Metabolism & Disease), 15 (10), special issue "Active Oxygen", 1978].
Enzymes capable of contributing metabolic deactivation of the above-mentioned active oxygen species, for example superoxide dismutases (SODs), catalase and glutathione peroxidase, vitamins having antioxidant activity, typically a-tocopherol (vitamin E), and the like are present in living organisms and their actions serve to keep the living organisms in a normal condition. However, it is not rare that, for one cause or another, the proper defense mechanisms in which such substances as mentioned above are involved become defective or the production of active oxygen species and the formation and accumulation of lipid peroxides occur to an extent that the capacity of the defense mechanisms is no more sufficient to deal the active oxygen species and/or lipid peroxides. In case some or other deficit is caused in the defense mechanisms in such a manner, peroxidation proceeds in the manner of chain reaction and causes various severe disorders in the living organisms. As typical examples of such disorders, there are mentioned various diseases due to platelet aggregation, inflammation, hepatopathy, arteriosclerosis, hemolysis, aging or senile diseases, retinopathy, pulmonary disorder, cardiopulmonary disorder due to certain drugs, and ischemic angiopathy [Jikken Igaku (Experimental Medicine), 4 (12), special issue for 1986 "Free radicals in living organisms and diseases"].
Drugs generally classifiable as antioxidants have been developed for the prevention and treatment of the above-mentioned diseases.